This invention relates to a jacket for use in supporting an offshore platform, and more particularly to a jacket for supporting a drilling platform of the type having a plurality of legs interconnected by a plurality of bracing members, a number of the bracing members having means for connection with the cables of apparatus for lifting the jacket during transportation to and positioning at an offshore drilling site.
In the past, offshore platforms have been employed advantageously in a number of different marine situations. For instance, offshore platforms have been employed as supports for radar or sonar stations, light beacons, and various types of marine laboratories. Furthermore, offshore platforms have frequently been employed in the exploration for oil in an offshore environment. Use of such platforms in the exploration for oil has received increasing emphasis as supplies of petroleum indigenous to the major industrial countries have diminished.
Such offshore platforms are usually supported by jackets which rest on the sea bottom. Typically, the jackets are massive structures consisting of a plurality (often four) of legs long enough to extend from the ocean floor to the water surface and arranged to form a tower-like structure which tapers inwardly from bottom to top and is held together by a plurality of bracing members and reinforcing struts. The jackets are fabricated onshore from sections of tubular and other structural steel, and thereafter are transported via barges or floatation units to the drilling site for positioning. To assist in the transportation and positioning, the jackets are frequently fitted with padeyes designed to provide sites for connecting the cables of lift apparatus such as a crane.
When the size of these jacket structures and the severity of the environment in which they are used are appreciated, it becomes apparent that there is a need for means for connecting cables to the jacket which will be strong enough to hold the weight of the jacket structure, yet simple enough to enable installers to rapidly connect and disconnect cables as required.
In this regard, one practice has been to simply weld padeyes at selected points on one or more sides of the jacket to receive hooks or shackles fixed to the end of the lift cables. While this procedure has been generally satisfactory, it is time consuming and expensive. Such permanently fixed padeyes must be correctly positioned and welded to the jacket during its fabrication. Difficulties may be encountered in orienting the padeyes such that stresses of a magnitude sufficient to shear the welds or damage the jacket during the transportation and positioning procedure are avoided. Furthermore, once the jacket is positioned on the ocean floor, the padeyes are no longer useful.
Lifting is typically accomplished using one or more main cables which branch off to multiple cables, each of which is connected to a padeye appropriately located on one side of the jacket. In this situation, in order to relieve undue shear stresses in the padeyes, each one must be oriented on a bracing member such that a line passing through the central axis of the branching member and through the point of attachment of the cable to the padeye is coincident with the axis of the cable while under tension lifting. If the installer achieves this optimum orientation, the padeye is subjected only to a tensile stress.
However, the orientation of the cable during lifting is dependent on the dimensions of the jacket, the location of the padeye, and the length of the cable used to connect with the main cable. Because of inherent imprecision in estimating the final cable orientation and the consequent difficulty inherent in orienting the permently-mounted padeye properly, the prior art padeyes have been susceptible to a variety of fabrication and assembly problems.